Leica recommends to discard batteries before they are 3 years old, from the manufacturing date not usage date. You can find the manufacturing date imprinted on the battery in week and year format. For instance if the number reads 5011 (my actual battery number) it means week 50, 2011. It would be interesting to know how old is your Leica battery.
M9 ZM 25/2.8 35/2 50/2 85/2